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Originally Posted by rmay635703
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The first case, sailing directly into the wind is easy to imagine, and as a result many windmill-powered boats have been built by enthusiasts. The windmill drives the wheels (or underwater propeller), which makes the craft move, which makes the apparent windspeed higher, which makes the craft move more forcefully,etc.
What makes the second case so interesting is that it seems so clearly impossible, (provided you haven't thought seriously about how it might be done). If you just turn the craft I described around and start downwind, as the craft speeds up, the apparent wind decreases, eventually to zero when the wind speed and craft speed are the same. There has to be some parasitic drag, so the craft can never get to wind speed... just as a sailboat cannot get to windspeed if sailing straight downwind, no matter how large the spinnaker.
The "trick" is, of course, that the propeller on Blackbird does not drive the wheels. The wheels drive the propeller. So the thing that makes the craft move, at first, is the total drag on the structure and propeller. The movement of the craft turns the wheels which turns the propeller in the direction opposite to that in which it would turn as a windmill. By moving, the propeller extracts energy from a larger piece of air.
Like any sailboat, the craft makes use of the
difference between speed relative to land (or water) and air. Without that difference, there is no energy harvested: cut a kite's string, and it falls to ground.
Sailing directly downwind faster than the wind has been demonstrated before (by a guy that worked for NASA, in the mid 1960's if I recall) but not so convincingly. The reason for such little interest has to do with utility, I suppose. Conventional (which now means equipped with rigid wings, etc.) land yachts are much faster on every point of sail other than directly down wind, and can sail down wind (tacking) and achieve a similar or better velocity made good... and it is almost certainly more fun to sail at 60 knots on tacks either side of directly downwind than it is to sail at 30 knots straight downwind. (Sailors of conventional slow sailboats might think: "Shouldn't he be saying 'gybing' downwind? But, in fact, fast boats tack downwind -- the wind moves over the bow, not the stern.)
The Physics of sailing in a river are interesting too, and prompt otherwise bright people to come up with all sorts of wrong answers for how sailing works -- just as there were many who maintained strenuously that a boat could not sail downwind at greater than windspeed.
Here is an interesting thread.
A related question is posed by Moronic Bumble in post 51 of that thread:
"A river runs straight from West to East at 10 knots. A 10 mile race is held: the boats sail downstream, from West to East. The first heat is held in the morning, when there is no wind. The second heat is held in the afternoon, when there is a 10 knot wind from the West. In which heat are the faster times recorded?"